The invention relates to a servo-control, in particular power steering for motor vehicles, having an input end or steering wheel end shaft part and a mechanism connecting this shaft part to a control end or steering gear end shaft part. The mechanism operates with a relatively large transmission ratio and is used to adjust a control slide of a servo-valve unit. In the case of rotational movements of the shaft part, the control slide is deflected from a central position by the mechanism. The direction and the amount of the deflection of the control slide is a function of the magnitude and direction of the rotational movement of the shaft parts. The deflection of the control slide connects a servo-motor, which is controlled by the servo-valve unit and drive-connected to the control or steering gear, to a pressure source so as to produce a forward or rearward motion with a setting force which depends on the magnitude of the displacement of the control slide.
In a power steering of this type, known from German Offenlegungsschrift No. 2,501,751, the mechanism is essentially formed by a steering nut which is arranged so that it can perform a screwing movement on a spindle arranged as part of the inlet end shaft part. The steering nut is arranged so as to be rotatable to a limited extent within an axially movable piston which follows the axial movement of the steering nut on the spindle and, by means of teeth provided on it, meshes with a toothed segment which in turn drives a linkage for controlling the wheels which steer the vehicle. A lever type protrusion radially penetrating the piston is firmly fixed to the steering nut and the free end of this protrusion actuates the control slide, arranged transverse to the axis of the steering nut, of a stationary servo-valve unit.
In this known servo-control, large setting motions of the control slide can be achieved. This offers the possibility of controlling servo-valve units with large opening cross-sections. It follows that a pressure source with a relatively low pressure is sufficient. The steering nut rotational motions actuating the control slide take place, however, against a relatively large frictional resistance which may reduce the steering sensitivity.
A power steering system is known from German Auslegeschrift No. 2,214,001 in which the mechanism is located between an input end or steering wheel end shaft part and a further shaft part on the same axis, which further shaft part is elastically connected to the input end shaft part. Gear wheels arranged on each of the shaft parts and meshing in turn with gear wheels on a common axis near the shaft parts, are used as the mechanism. Between the latter gear wheels, there is a lever mechanism whlch converts relative rotations between the latter gear wheels into a translational motion in the axial direction of these gear wheels in order to adjust the control slide located on the same axis as the last mentioned gear wheels. Although this arrangement can, in appropriate case, achieve the result that the setting distance of the control slide is large in comparison with the relative rotations between the shaft parts, the size of the arrangement is undesirably large.
Power steering systems are also known from DE-OS No. 32 48 251, for example, in which systems the control valve unit is directly located on one shaft part and the other shaft part is directly connected to the control slide or control slides of the valve unit. Although this permits a very compact installation, the setting distances of the control slides are small corresponding to the very small relative rotations between the shaft parts. This has the result that the control valve unit can only control openings with a relatively small cross-section because otherwise the setting distance of the control slide would not be sufficient to effect control in every condition between fully open and fully closed. Because of the small cross-sections, however, a pressure source with a relatively high pressure must be used in order to supply the necessary quantity of the fluid medium, generally a hydraulic medium, to the servo-motor, despite the throttling effect of the relatively narrow cross-sections of the control valve unit. This implies that the pressure source, for example a pump, has a relatively high power requirement which, in the case of a power steering system, has to be taken from the vehicle engine and is therefore not available for actually propelling the vehicle. For vehicle engine fuel consumption reasons, however, it is desirable that the auxiliaries should have the smallest possible power requirement.
It is an object of the invention then, to produce a servo-control or power steering system which features small space requirements, relatively low power demands on the pressure source and low friction within the mechanism.
The invention achieves this object for a servo-control or power steering system of the type mentioned in that the mechanism is located directly between the input end shaft part, or a part directly connected to it, and a further shaft part, or a part directly connected to it. The mechanism is elastically connected on the same axis as the input end shaft part. The servo-valve unit is located on a shaft part or a part connected to it. The mechanism, a setting lever directly actuating the control slide, is rotationally supported on a shaft part, or the part directly connected to it, by means of a support pin located parallel and eccentric to the shaft axis. The lever is hinge-connected or coupled to the other shaft part, or the part directly connected to it, on the support pin side of the shaft axis and at a radial distance from the support pin.
According to a preferred embodiment of the invention, therefore, the mechanism is effected by a single lever element located between the shaft parts or the parts directly connected to them. This readily permits a high transmission ratio because the setting lever is hinge-connected or rotatably coupled near its support pin on one shaft part, or the part connected to it, to the other shaft part, or the part connected to it. The invention is therefore based on the general idea of locating a compact and, from the design point of view, extremely simple mechanism with a large transmission ratio axially directly between the shaft parts so that the servo-valve unit can be arranged on a shaft part or the part connected to it in a known manner and the control slide or control slides can nevertheless execute a large setting movement compared with the relative rotational motion of the shaft parts. It follows that the control slides can control large opening cross-sections with relatively small throttling resistances so that the pressure source or pressure pump of the servo-control or power steering system only has to operate against a relatively small resistance and therefore has a greatly reduced power requirement.
According to one preferred embodiment of the invention, the setting lever is a double arm lever with a support pin parallel and eccentric to the shaft axis. The double arm lever is rotationally supported on one shaft part, or the part directly connected to it, and has one lever arm hinge-connected to the control slide and the other lever arm hinge-connected to the other shaft part or to the part directly connected to it. In this arrangement, the arm of the double lever actuating the control slide is preferably longer than the other lever arm. However, even with the same length of lever arms, a high transmission ratio is achieved if the support pin is located with a large eccentricity relative to the shaft axis.
According to another embodiment of the invention, the setting lever has a limited pivoting range. By this limiting arrangement, the relative rotation possible between the shaft parts is limited and, at the same time, positive mechanical coupling is provided between the shaft parts so that the servo-control or power steering remains functional even in the case of defects in the fluid system. The limiting of the pivoting range thus provides mechanical security which, if necessary, maintains a purely mechanical drive connection between the shaft parts therefore, between the handle or steering wheel and the control or steering gear.
In order to limit the pivoting range, the setting lever can, by means of a recess radially outward from its support pin, encompass with play a protrusion on one shaft part or on the part directly connected to it.
According to another embodiment of the invention, the setting lever is restrained in a central position by spring means. For this purpose, a spring connection, for example by means of a torsion rod, can in principle be located between the shaft parts.
In order that the arrangement of the setting lever should have sufficient space, however, the setting lever is preferably restrained in the central position by means of a C-type spring clip which encompasses the setting lever or one of its lever arms and the abutments of which are located on a shaft part, or the part directly connected to it, at a distance apart corresponding to the width of the setting lever or lever arm in the region of the ends of the spring clips. In the central position of the setting lever, therefore, the ends of the spring clip are in contact both with the flanks of the setting lever or lever arm and with the abutments whereas, when the setting lever is deflected, the C-type spring clip is increasingly spread out, one end of the clip being in contact with one of the abutments and the other end of the clip being in contact with the opposite flank of the setting lever or lever arm.
The control slide is preferably arranged as a piston-type part displaceable in a radial plane of the shaft. In view of the relatively large setting distances provided by the invention, the control slide is preferably arranged with its longitudinal axis intersecting the shaft axis at right angles.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings(s) which show, for purposes of illustration only, an embodiment in accordance with the present invention.